The Uniformitarian Principle (UP) in linguistics is the application of a key principle of science - uniformitarianism - in linguistics. As applied in linguistics,
it states that unless we can demonstrate that the conditions of language acquisition or use have changed between some time in the past and the observable present, we have to assume that the same types, range, and distribution
of linguistic structures and linguistic changes occurred at that time in the past as we can observe in the present.

If language was normally acquired in the past in the same way as
it is today - usually by native acquisition in early childhood -
and if it was used in the same ways - to transmit information, to
express solidarity with family, friends, and neighbors, to mark one's
social position, etc. - then it must have had the
same general structure and organization in the past as it does
today, and it must have changed in the same ways as it does today.

The UP does not say that particular linguistic details have
been uniform over time. For instance, from the fact that English now
has interdental fricatives (as in thorn,that), it does
not follow that all its ancestors also did; from the fact that Rajasthan
is now very diverse dialectally, it does not follow that it always has
been; and so on.

The UP does not say that if we cannot demonstrate
that a language has changed, we assume that it has not changed.
On the contrary, since all contemporary languages in which we have
looked for change in progress actually do show change in progress, and
since all languages documented for more than a few centuries show
change, we have to assume that language change is somehow inherent to
human language and thus universal. That is a valid inference from the
UP.

The UP does not say that a language does not change unless
change is caused by contact with other languages, and it also
does not say that a language does not change unless external
forces make it change. Those are widespread assumptions, but the facts
show that they are wrong. For instance, Icelandic, which has been
effectively isolated for centuries, has undergone dramatic changes in
pronunciation and syntax since the 13th century. (Its inflectional
morphology has changed little, and Icelanders like to boast about that;
but if the language were really resistant to change, the phonology and
syntax should not have changed either.)

The UP does not say that language does not change if the
process of acquisition has not changed. On the contrary, it appears that
the universal process of (imperfect) acquisition is a principal cause of
change.

If external conditions were the same in the past as they are in the
present, then the internal structure of human language must have been
generally the same in the past as it is in the present. The same goes
for speech communities and language change. Human language is a single
phenomenon and always behaves the same under the same conditions.

If the forces determining language structure and driving language change
have not been altered, then we expect to find the same kinds of
structures and changes.